32 bit operating systems can only address 3gb of ram. 64 bit OS's can support more than that.

32 bit Vista will still only be able to support 3 gigs of ram.

What does this mean ? Having 3GB\4GB RAM will endup as 3GB for Vista ?

If yes ? Any reason why so ?

32Bit Operating systems only utilize up to 3gb of RAM. If your computer has more then that it won't register within the OS itself making the purchase of additional RAM pointless unless you plan to upgrade to 64bit.